Length : The length field occupies the first byte and indicates the total number of bytes (excluding the length field itself) in the TPDU.

Fixed Parameters : The fixed parameters field contains parameters, or control fields that are commonly present in all transport layer packets. It consists of five parts: Code, source reference, destination reference, sequence number, and credit allocation.

Code : The code identifies the type of the data unit for example, CR for connection request or DT for data.

CR : Connection request
CC : Connection confirm
DR : Disconnect request
DC : Disconnect Confirm
DT : Data
ED : Expedited data
AK: Data acknowledge
EA : Expedited data acknowledge
RJ : Reject
ER : Error

Source and destination reference : The source and destination reference fields contain the addresses of the original sender and the ultimate destination of the packet.

Sequence Number : As a transmission is divided into smaller packets for transport, each segment is given a number that identifies its place in the sequence. Sequence numbers are used for acknowledgement, flow control, and reordering of packets at the destination.

Credit Allocation : Credit allocation enables a receiving station to tell the sender how many more data units may be sent before the sender must wait for an acknowledgement.

Variable parameters : It contains parameters that occur in frequently. These control codes are used mostly for management.

Data : It may contain regular data or expedited data coming from the upper layers. Expedited data consist of a high priority message that must be handled out of sequence. An urgent request can supersede the incoming queue of the receiver and be processed ahead of packets that have been received before it.

Another way of looking at the transport layer is to regard its primary function as enhancing the QoS (Quality of Service) provided by the network layer. If the network service is impeccable, the transport layer has an easy job. If however, the network service is poor, the transport layer has to bridge the gap between what the transport users want and what the network layer provides.

The transport service may allow the user to specify preferred, acceptable, and minimum values for various service parameters at the time a connection is setup.

Now we will discuss some of the QoS parameters

The connection establishment delay is the amount of time elapsing between a transport connection being requested and the confirmation being received by the user of the transport service. It includes the processing delay in the remote transport entity. As with all parameters measuring a delay, the shorter the delay, the better the service.

The connection establishment failure probability is the chance of a connection not being established with in the maximum establishment delay time, for example, due to network congestion, lack of table space some where, or other internal problems.

The Throughput parameter measures the number of bytes of user data transferred per second, measured over some time interval. The throughput is measured separately for each direction.

The Transit delay measures the time between a message being sent by the transport user on the source machine and its being received by the transport user on the destination machine. As with throughput, each direction is handled separately.

The Residual error ratio measures the number of lost or garbled messages as a fraction of the total sent. In theory, the residual error rate should be zero, since it is the job of the transport layer to hide all network layer errors. In practice, it may have some finite value.

The Protection parameter provides a way for the transport user to specify interest in having the transport layer provide protection against unauthorized third parties (wire tapers) reading or modifying the transmitted data.

The priority parameter provides a way for a transport user to indicate that some of its connections are more important than other ones. And in the event of congestion, to make sure that the high-priority connections get serviced before the low-priority ones.

Finally, the Resilience parameter gives the probability of the transport layer itself spontaneously terminating a connection due to internal problems or congestion.

The QoS parameters are specifies by the transport user when a connection is requested. Both the desired and minimum acceptable values can be given . In some cases, upon seeing the QoS parameters, the transport layer may immediately realize that some of them are unachievable.